offset method
Implementation
@override
void offset(int index) {
int i = _cells.indexWhere((element) => element.index == index);
if (i < 0) {
i = 0;
}
for (int a = _cells.length - 1; a >= i; a--) {
removeAt(_cells[a].index + 1);
putAt(_cells[a].index + 1, _cells[a].data);
}
_cells.removeAt(i);
}